1. Field of the lnvention
This invention relates to a mixing ratio detecting device which is capable of opticalIy deteoting a mIxing degree of a liquid consisting of two components, and particularly concerns to a mixing ratio detecting device suited for detecting a mixing degree of gasoline and alcohol.
2. Description of Prior Art
In an internal combustion engine, a liquid fuel consisting of gasoline and alcohol has been suggested to adopt so as to alleviate a controversial air-pollution. In the internal combustion engine of this type, it is necessary to detect a mixing ratio of the gasoline and alcohol so as to determine an optimum timing of ignition. In cope with the requirement, a mixing ratio detecting device has been introduced in which a transparent column is provided, a bottom of which is in contact with a mixing liquid of gasoline and alcohol. A light emitting diode (LED) and a photo diode is disposed in a manner to sandwich the column. Light beams from the LED passes within the column, and impinges on an interface between the column and the mixing liquid at more than a critical angle. The light beams, thus impinged on the interface at more than the critical angle, totally reflect to escape from the column so as to fall on the photo diode which generate an output in accordance with an quantity of the light beams received. In this instance, the quantity of the light beams which the photo diode receives increases with the increase of the alcohol component because the light beams from the LED lengthen its optical path to impinge on a relatively upper part of the photo diode.
However, the increased alcohol component causes to saturate output characteristics of the photo diode as shown in FIG. 6 in which depicts a graph with abscissa and ordinate as a mixing ratio and an output from the photo diode respectively. This is because the light beams from the LED diverge right and left, and the diverging degree increases as the optical path lengthens which is equivalent to a travel length required for the light beams to reach the photo diode throught the column.
This requires a complicate procedure to treat the output from the photo diode in obtaining a precise mixing ratio of gasoline and alcohol, and rendering it difficult to carry out a temperature compensation easily.
Therefore, it is an object of the invention to provide a mixing ratio detecting device for composite liquid which is capable of easily compensating for output characteristics from a photo diode, and always securing a precise mixing ratio with a simple construction.